1. Field of the Invention
This invention relates to probe system for use with semiconductor wafers. More particularly, it relates to a probe system having an upward facing probe head disposed to interface with the top surface of a wafer-under-test with the wafer held upside down by a wafer support chuck. Said probe system including an evenly spring-supported, gas piston push-pulled, upward-facing probe head, wherein one or more mercury electrodes of various shape and area can be formed to make low-pressure uniform and clean contacts in the upward facing orientation with the upside down held wafer-under-test. Said probe head, being evenly spring-supported, easily adopts itself to the top surface of the wafer, which may have some inclination. Said probe system is vertically push-pulled by a gas piston for probing the surface of the wafer with consistent force regardless of the thickness of the wafer.
2. Description of the Background Art
Four-point probe systems are commonly used in device fabrications for measuring and mapping sheet resistivity of ion implanted layers and metal film depositions of a blank test semiconductor wafer during the production of integrated circuits (ICs). However, the ion implanted or infused layer of the most advanced ICs are so shallow that a significant portion of test current injected from one of the needles of the four-point probe to such layer can easily leak into the opposite type substrate. This uncontrollable test current leakage can cause deterioration of measurement repeatability and accuracy to the ion implanted layer. Penetration of the probe needles through the implanted layer may also cause the same measurement repeatability problems. These are becoming a greater problem as the geometry of semiconductor devices gets smaller and smaller as technology advances.
Also, if a first test was performed on a wafer with a metal film, and the next test to be performed is on an ion-implanted wafer, it usually is necessary to change the probe head to get the best measurement results and avoiding cross contamination. Previously, changing probe heads has been done manually for all four-point probe systems. Even in a fully automatic cassette-to-cassette system, there is only one probe head that is installed in the system, and there is no provision for automatically changing that single probe head as the types of tests that need to be performed vary. There too that probe head must be changed manually. Changing probe heads is not only time consuming, but also makes it easy to introduce contaminating particles into the system's mini-environment. Furthermore, mistakes may also happen in changing the probe heads.
U.S. Pat. No. 6,435,045 issued Aug. 20, 2002, entitled Apparatus and Method for Automatically Changing the Probe-Head in a Four-Point Probe System (co-owned by the same entity as the current application) discloses a system that automatically changes the probe-heads between tests thus overcoming cross-contamination and errors in changing of the probe heads. This system uses probe needles and thus is less desirable for the newer, smaller geometry semiconductor devices that are becoming increasingly popular for the reasons outlined above.
Severin et al. (U.S. Pat. No. 3,794,912 issued Feb. 26, 1974) disclosed a basic four-point probe that includes mercury contacts to a semiconductor wafer. This system contains no safeguard on the handling of the mercury and is not an automatic system. Greig (U.S. Pat. No. 4,101,830 issued Jul. 18, 1978) disclosed a non-four point probe system that utilizes mercury contacts to a semiconductor device. This system also is not automatic and has only marginal safeguards on the handling of mercury.
Shulman (U.S. Pat. No. 4,521,730), Lederman (U.S. Pat. No. 4,409,547), and Mazur et al. (U.S. Pat. No. 5,036,271) each disclose a mercury probe system that interfaces with the wafer right side up and the probes extending downward. Additionally, each of the above-listed patents further disclose, respectively, a method for refreshing the mercury in the probe, the suggested use of springs to bias the mercury probe head to the wafer, and a setup for enabling automatic mercury probing. However, in addition to not disclosing a mercury probe system with the probes extend upward, the mercury probes disclosed in each of those patents cannot facilitate multiple mercury contacts in various shapes and area sizes to engage the wafer-under-test.
The present invention overcomes all of the above problems with the prior art as will be seen in the following description of the present invention.